This fight actually happened, although only in narrative, it was during the Murray Woroner produced All-Time Heavyweight Championship aired in 1967 when Marciano and Tunney were matched in the first round. Many believed that Rocky would not make it through against such a ring savy boxer. Yet make it through he did, but what really made everyone sit up was the one sided nature of the transistorized battle for Marciano won on points and not just close. Rocky, said the computer, won almost every round.
There is speculation that the reason for Marciano's complete superiority was more to do with the fact that of all the extant heavyweight champions involved in the fictitious tourney Gene Tunney was the only one who refused to be interviewed and used as part of the shows that were pieced together to make coherent and almost believable events...This may just be another example however of a side swipe at Marciano himself as there are many self appointed boxing historians who could not, and still can't, grasp that the reason Marciano won all his fights was not because of his opponents deficits but because of Marciano's strengths...Thanks for the posts and allowing me to reply.

all it takes with marciano is one right hand, 15, even 10 rounds inside a ring with rocky is a long time for someone to not make a mistake. tunney was a great boxer, i do believe he'd be well ahead on points, possibly even winning every round but at some point rocky will catch him and KO him

0:00 The fight gets underway as the bell sounds the start of Round 1.
0:09 Tunney traps Marciano in the corner!
0:09 Each fighter is measuring the other. Now Tunney lets his hands go and scores several quick punches to the head of Marciano.
0:25 Marciano works free! The fighters move to ring center.
0:37 Marciano lets his hands go but his punches land on the arms of Tunney.
0:40 Some good body work by Tunney but not much on those punches.
0:57 Tunney tries to drop the right hand over the top but Marciano ducks under it.
01:09 Marciano connects with a violent cross!
01:09 Tunney takes a step back, obviously hurt!
01:19 Both fighters come into this fight predicting a win... Not that you'd expect anything else.
01:31 Tunney misses with that jab.
01:38 Tunney has Marciano blocked in!
01:43 Tunney is holding Marciano tighter than most men do their wives -- or girlfriends!
01:55 Lane is shouting at Tunney to break!
01:59 Tunney feels his back touch the ropes and immediately moves away. He doesn't want to get trapped there.
01:59 Tunney's finds a home on Marciano's jaw.
02:21 Marciano misses with what looked like a backhand. He's lucky. If that had landed, he might have drawn a warning.
02:28 Tunney lands a devastating cross!
02:28 Marciano is trying to reach for the ropes! His legs have turned to jelly!
02:37 Marciano pins Tunney in the corner!
02:42 Marciano lands a devastating cross!
02:42 Tunney was stunned by that blow!
02:51 These guys are throwing caution to the wind as they go at it blow for blow.
03:00 The fighters work their way back to the center of the ring...
03:00 The crowd is literally screaming in approval as the two fighters look for a minutes respite after a sizzling round of action.

Round 2

0:00 Lane motions the fighters together as the bell sounds to begin Round 2.
0:11 Tunney is motioning with his gloves for Rocky Marciano to bring it on, but Tunney is not being overly aggressive himself right now.
0:16 Marciano slides into the corner...
0:16 Tunney is trying to put his punches together and he tags Marciano with a nice combination of punches.
0:28 They're coming back to the center of the ring now...
0:28 Tunney lands a rapier like jab to the head of Marciano.
0:41 There's a hard shot off the top of Marciano's head!
0:41 Marciano's eye's are glazed over after that shot, he is definitely hurt!
0:50 There's a lot of excitement surrounding this fight. The fans are really in to it.
0:59 Tunney just can't find the range with that jab.
01:17 Marciano fires a left hand that Tunney takes high on the shoulder.
01:19 Gene Tunney is trying to move in behind the jab, which lands.
01:28 Tunney continues to pop the jab.
01:37 As high as Marciano is holding his gloves he is leaving his body wide open to a damaging body punch if Tunney takes advantage of it.
01:46 Tunney is on his bicycle, moving from corner to corner forcing Marciano to chase him.
01:54 Tunney scores with a beautiful cross that just missed landing flush on Marciano's jaw.
02:04 Marciano backs into the near right corner.
02:08 Tunney lands a strong cross!
02:08 Marciano is just looking for a place to fall after that shot!
02:20 Marciano eats a jab from Tunney.
02:35 Tunney is dancing and flicking harmless punches at Marciano.
02:41 Tunney straightens Marciano up with the impact of that uppercut to the head.
02:51 Tunney goes to the head, then digs a nice combination to the body that had something on it.
03:00 And there's the bell ending Round 2.

Round 3

0:00 It's Round 3 of this scheduled 15 rounder.
0:09 Marciano moves into a neutral corner.
0:14 Tunney misses with the left and then fails to connect with the right as well.
0:23 Marciano's corner is urging him on. They're really vocal tonight.
0:29 Tunney catches Marciano with an uppercut as he tries to duck.
0:39 Tunney moves into the far left corner.
0:39 Marciano likes to use the hook but he'd better get a little closer when he throws it or Tunney's going to beat him to the punch and hurt him!
0:53 Tunney takes a hard combination from Marciano.
0:53 That blow connected solidly to the head of Tunney and he is stunned!
01:03 Tunney's jab falls short, but the cross gets in behind it.
01:14 Tunney fires a jab to the body.
01:31 Tunney is clinching. Lane better watch because some fighters aren't above doing some nasty head work when they're in close like this.
01:35 Tunney shoves Marciano away.
01:45 Marciano looks to land a jab but misses the mark.
01:54 Tunney eats a short cross from Marciano amd backs away.
02:07 Tunney usues his legs to avoid Marciano's punches.
02:10 Tunney gets the cross through Marciano's guard.
02:31 Rocky Marciano fires away but finds nothing but air.
02:42 Marciano is staying at the center of the ring.
02:46 Rocky Marciano scores with a tremendous right just above the waistline!
02:46 Those punches hurt Tunney and he looks to hang on.
03:00 Marciano fires a wild overhand right and misses everything.
03:00 We're at the 3:00 mark and that brings this round to an end.

Round 4

0:00 Both fighters look ready to go as Round 4 begins.
0:14 Rocky Marciano drives home a wicked hook to the midsection!
0:14 Tunney is gasping for air!!
0:19 Both fighters are unmarked at this stage of the fight.
0:23 Tunney got some great advice from Eddie Inabagsky between rounds, let's see if he puts it to use here in round 4.
0:23 Gene Tunney's jab is landing with some authority.
0:35 Tunney throws an uppercut from way outside. It lands but he's lucky! That's a dangerous punch to throw unless you're in close.
0:45 Rocky Marciano drives the hook home.
0:54 Rocky Marciano retreats into a neutral corner.
0:59 Marciano takes a heavy hook downstairs.
0:59 Marciano is reeling..
01:14 Tunney is trapped in the corner...
01:19 Marciano cranks up the hook and connects to Tunney's jaw, that one had bad intent behind it!
01:19 Gene Tunney was stunned by that blow!
01:32 Marciano lands a smashing hook!
01:32 Tunney is hurt!
01:36 There seems to be a bit of puffiness under Tunney's right eye.
01:45 Gene Tunney picks off the cross by Marciano.
01:52 That cross by Marciano landed with authority. His glove nearly disappeared in Tunney's stomach!
01:52 Marciano's last punch has given him a chance to end the fight this round. He's got Tunney on Queer Street. Now, can Marciano finish him?
02:05 Marciano just delivered a wake-up call!!! He really caught Tunney with a sharp combination!
02:05 Tunney was stopped cold by that series of shots!
02:19 Oh!!! Marciano unleashed a crippling right hand to the head!
02:19 Tunney is badly hurt!
02:32 Marciano drops his shoulder as if to throw a right, then quickly rips a quick left hook to the body.
02:32 Tunney was hurt by those blows!
02:37 Tunney is exhausted. Lane is waving his arms! He's stopping the fight! Marciano is the winner on a TKO!

Fight Result

We go to the ring where Chuck Hull is ready with the call...
Ladies and Gentlemen! The winner by technical knockout at the 02:38 mark in Round 4 is...

I think Tunney puts on a masterful boxing performance and looks Marciano look really poor until about round 11.

At this point Marciano has maybe won 3 rounds maximum but, as the fight wares on his consistent pressure and hard punching to Tunneys arms and shoulders mean Tunney struggles to keep his performance up to scratch. As Marciano lands more and more and begins to press Tunney into corners and against the ropes.

Tunney tries to go evasive to simply survive the last few rounds but, can't keep Rocky off him and eventually gets knocked out.